1. Field of the Invention
This invention relates to a multi-speed transmission for a vehicle, which is provided between a prime mover and a driven wheel in a vehicle such as an automobile.
2. Description of the Related Art
A vehicle is normally provided with a transmission between a primer mover and driven wheels. One such known transmission is a multi-speed transmission that is used to achieve a plurality of predetermined gear ratios or gears. Planetary gear type multi-speed transmissions which use a plurality of planetary gear sets and engagement elements such as clutches and brakes to engage the elements that form those planetary gear sets together are widely used. For example, Published Japanese National Phase Application No. 2003-514195, JP(A) 2002-206601, JP(A) 2002-213545, JP(A) 2002-227940, JP(A) 2002-266956, and JP(A) 2002-295609 propose a multi-speed transmission which has a first transmitting portion which includes two front planetary gear sets and a second transmitting portion which includes two rear planetary gear sets and can achieve multiple speeds of seven or more forward gears. In particular, the multi-speed transmission disclosed in Published Japanese National Phase Application No. 2003-514195 has two non-switchable front planetary gear sets and a plurality of engagement elements, and is able to achieve at least seven forward speeds (i.e., gears) by selectively engaging those engagement elements.
When further increasing the number of speeds (i.e., gears) of the multi-speed transmission for a vehicle, it is desirable that the gear ratio steps (i.e., the change ratio of the gear ratios of consecutive gears) be equal or near thereto. With this kind of conventional multi-speed transmission, however, the gear ratio steps of the gears are not always good. For example, as illustrated in FIG. 1(B) of Published Japanese National Phase Application No. 2003-514195, the gear ratio step between first gear and second gear is 1.86, which is a large step. There is also a fair amount of variation in the sizes of the gear ratio steps, which is undesirable. There has therefore been a need for the development of a multi-speed transmission for a vehicle that can achieve multiple speeds (i.e., gears) while maintaining good balance among the gear ratio steps between gears.